Or alternatively, maybe it's true that there are major economies of scale to most businesses, and big companies get big not through "lobbying" and "regulation" but because they're far more efficient than small companies?
Yes, they are efficient and well run but they also can suppress threatening innovation. At some point big companies can buy out any potential competition. You can see that in tech where a lot of startups don't even try to run a profitable business but plan on getting bought out. Same in medical tech where startups just do basic research that's sold to the big guys.
Or at least, having one big company serving 100 people can be more efficient than 10 smaller companies delivering 10 people each a similar service.
Is this really more efficient for society and innovation in the long run? It just cements the status quo.
This makes sense, but ask yourself why a search engine company and a phone software developer should be part of one big company. Vertical integration has almost no justifiable reason to be protected, it's primary purpose is for locking you into an ecosystem, which gates out new, innovative players.